1. Field of the Invention
The present invention relates to an X-ray generator apparatus having an X-ray tube which generates X-rays when applied with a high voltage obtained by increasing an input voltage to a high A.C. voltage of a high frequency by means of a frequency converter and a high voltage transformer or the like and rectifying the high A.C. voltage.
2. Description of the Related Art
An example of this type of conventional X-ray generator apparatus is shown in FIG. 1 . In order to enhance the performance and make the device small and lightweight, a frequency converter 2 for converting the frequency of a voltage supplied from an input power source (A.C. power source) into a high frequency is connected to the primary winding of a high voltage step-up transformer 3. An output voltage of the frequency converter 2 is increased by the high voltage transformer 3 and an output voltage of the high voltage transformer 3 is rectified by a high voltage rectifier 4. A rectified output of the high voltage rectifier 4 is applied between an anode and a cathode of an X-ray tube 5 serving as an X-ray source.
The frequency converter 2 is generally formed of a rectifier for converting the input A.C. voltage into a D.C. voltage, a capacitor for filtering the D.C. voltage, and an inverter for converting the D.C. voltage from the capacitor into an A.C. voltage of a desired frequency. The frequency converter 2 converts the frequency fo (which is a commercial frequency and is generally 50/60 Hz) of the input A.C. voltage to a frequency f1 which is higher than the frequency fo and then applies the voltage to the high voltage transformer 3.
As the output frequency f1 of the frequency converter 2 is set to be higher, the size and weight of the frequency converter 2 and high voltage transformer 3 can be reduced. Since the impedances of coils and capacitors generally vary according to the frequency, the capacitance and inductance can be reduced as the frequency is set higher if the impedances are kept unchanged. Since the capacitance and inductance vary in proportion to the size of the capacitor and coil, the size and weight of the frequency converter 2 and high voltage transformer 3 using the coil and capacitor can be reduced as the frequency becomes higher.
However, in the above X-ray generator apparatus, the output frequency f1 of the frequency converter 2 cannot be increased limitlessly and the upper limit thereof is determined by the characteristic of the high voltage transformer 3 for the following reason.
FIG. 2 shows an equivalent circuit diagram of the device shown in FIG. 1 in view of the secondary winding portion of the transformer 3. In FIG. 2, L1, L2, and M respectively denote the primary inductance, secondary inductance, and mutual inductance of the high voltage transformer 3. N denotes the turn ratio (the number of turns of the secondary windings/the number of turns of the primary windings) of the transformer 3. In this case, in order to obtain a high output voltage, the high voltage transformer 3 is so designed that the number of turns of the secondary winding must be made very larger than that of the primary winding, and as a result, the secondary inductance L2 becomes very larger than the primary inductance L1 and mutual inductance M. Therefore, the inductance of the secondary portion of the high voltage transformer 3 which is actually equal to (L2-M) as shown in FIG. 2 can be regarded as being equal to the secondary inductance L2 by neglecting M. Therefore, in the following explanation, it is assumed that the inductance of the secondary portion is equal to L2. Further, if the equivalent impedance of the X-ray tube 5 is Rx, the terminal voltage of the X-ray tube 5 is Ex, and the rectifier 4 is omitted from being consideration since it does not relate to the terminal voltage Ex, then the secondary inductance L2 is connected in series to the impedance Rx.
If the output frequency of the frequency converter 2 is f1, an impedance Z2 due to the secondary impedance L2 can be expressed by the following equation and it is understood that it varies in proportion to the output frequency f1 of the frequency converter 2: EQU Z2=2.pi..P.f1.L2 (1)
Further, the voltage Ex applied to the X-ray tube 5 is expressed as follows: EQU Ex=E2.Rx/(Rx+Z2) (2)
Since the turn ratio N is very large and thus the inductance (L1-M)/N.sup.2 can be neglected, the terminal voltage E2 of a mutual inductance M is expressed as follows using the output voltage E1 of the frequency converter 2: EQU E2=E1. N (3)
As is clearly understood from the equations (1) and (2), the impedance Z2 becomes higher as the output frequency f1 of the frequency converter 2 becomes higher, causing a problem that the voltage Ex applied to the X-ray tube 5 is lowered. For this reason, the output frequency f1 of the conventional frequency converter 2 has an upper limit of several tens of KHz and a higher frequency exceeding this upper limit cannot be attained. If the frequency is set to several tens of KHz, it is difficult to reduce the size and weight of the transformer and rectifier circuit and audio noise may be generated from the transformer 3.
The reason the output frequency f1 of the frequency converter 2 can be increased only to several tens of KHz at most is that the secondary inductance L2 of the high voltage transformer 3 is very large.
In order to solve the above problem, it has been proposed to modify the primary portion of the high voltage transformer 3 as shown in FIGS. 3 and 4. In the circuit of FIG. 3, a capacitor C1 is connected in series to the primary winding of the high voltage transformer 3 to form a series resonance circuit in the primary portion. In the circuit of FIG. 4, a capacitor C2 is connected in parallel with the primary winding of the high voltage transformer 3 to form a parallel resonance circuit in the primary portion. However, in either circuit, a voltage of the primary portion of the high voltage transformer 3 is equivalently increased by the series resonance or parallel resonance circuit. The inductance L1 of the primary portion is originally small and the resonance voltage is low, and therefore, in order to obtain the same voltage applied to the X-ray tube 5 as that obtained in a case wherein no resonance circuit is connected, it is only possible to increase the output frequency of the frequency converter 2 to several times the output frequency set in a case wherein no resonance circuit is connected.
Further, in U.S. Pat. No. 4,545,005 (Mudde), the secondary winding of the high voltage transformer is divided into a plurality of sub-windings to increase the operation frequency of the high voltage transformer. The sub-windings are connected in series through bridge rectifier circuits. The outputs of the rectifier circuits are coupled in series and applied to an X-ray tube. Every other sub-windings have the same sense and the rest have the opposite sense. However, the core of the high voltage transformer is not divided and thus the high voltage transformer can be regarded as being a single transformer. The capacitance of the secondary winding of the transformer can be reduced but the inductance thereof cannot be reduced by a division of the secondary winding. In this USP, an output of one frequency converter is simply connected to a single high voltage transformer. Therefore, like the conventional case shown in FIG. 1, it is only possible to increase the frequency to several tens of KHz at most.
Further, in U.S. Pat. No. 4,317,039 (Romandi), a frequency converter is formed of a main rectifier for three-phase current, a filter member, and a plurality of inverters connected in parallel to the filter member. The outputs of the plural inverters are respectively supplied to a plurality of transformers. The outputs of the plural transformers are respectively supplied to a plurality of rectifiers. The rectified voltages are added together and applied to the X-ray tube. The plural inverters are controlled by a control circuit in such a manner that the phases of the output voltages are chronologically offset relative to one another. The phase displacement amounts expediently to 90.degree., so that the ripple of the high-voltage at the X-ray tube, as compared with the instance in which only a single inverter and a single transformer are provided, is reduced by a factor of the number of converters used. In this USP, the frequency converter outputs plural different phase voltages to be applied to the plural transformers. If there is any capacitance between an output rectifier and the X-ray tube, the ripple of the output voltage can be made small by increasing the frequency of the output voltage without by displacing the phases of the output voltages. A high-voltage rectifier means of this USP comprises voltage doubler circuits connected to the secondary windings of the transformers. The frequency doubler circuit includes capacitors. Moreover, a high-voltage cable transmitting the high-voltage from the high-voltage rectifier means to the X-ray tube also includes capacitors. The ripple of the output voltage can be smaller by increasing the frequency of the output voltage than displacing the phases of the output voltages. Therefore, this USP in which the phases of the output voltages are displaced in order to make small the ripple does not aim to increase the frequency. This USP describes that the feed frequency of the high voltage transformers, i.e., the frequency of the inverters lies in the medium frequency range and amounts to approximately six to seven KHz. In this UPS, the number of the inverters must be plural in order to displace the phases of the output voltages.